Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation.

Isoflavones are secondary plant constituents of certain foods and feeds such as soy, linseeds, and red clover. Furthermore, isoflavone-containing preparations are marketed as food supplements and so-called dietary food for special medical purposes to alleviate health complaints of peri- and postmenopausal women. Based on the bioactivity of isoflavones, especially their hormonal properties, there is an ongoing discussion regarding their potential adverse effects on human health. This review evaluates and summarises the evidence from interventional and observational studies addressing potential unintended effects of isoflavones on the female breast in healthy women as well as in breast cancer patients and on the thyroid hormone system. In addition, evidence from animal and in vitro studies considered relevant in this context was taken into account along with their strengths and limitations. Key factors influencing the biological effects of isoflavones, e.g., bioavailability, plasma and tissue concentrations, metabolism, temporality (pre- vs. postmenopausal women), and duration of isoflavone exposure, were also addressed. Final conclusions on the safety of isoflavones are guided by the aim of precautionary consumer protection.

Pathophysiological relevance of selenium.

The essential trace element selenium (Se) is a central constituent of 50-70 selenoprotein variants encoded by 25 human genes. Incorporation of the 21st proteinogenic amino acid selenocysteine occurs cotranslationally and requires peculiar features of the corresponding mRNA, a dedicated tRNA and a complex translational machinery for decoding UGA in this context. Thyroid hormone (TH) synthesis and protection of the thyroid gland from H2O2 and reactive oxygen species derived therefrom as well as TH activation and inactivation by deiodinase enzymes requires Se. Altered Se status has been associated with benign (goiter and autoimmune thyroid disease) and malignant thyroid maladies and several but not all Se supplementation studies reported on beneficial effects. Whether adequate or therapeutic Se supply is advantageous for the functional unit of the thyroid, the angiofollicular unit, or for the immune system or for both requires more controlled clinical trials and further in vitro and animal experimental studies. Development of proper diagnostic tests to monitor Se status of the thyroid gland and identification and validation of clinically useful thyroid-related biomarkers for Se action on the largest human endocrine gland are required. Several knockout and transgenic mouse models have revealed valuable insight into the role of Se and selenoprotein function for the thyroid. Recently, first human mutations in genes encoding a selenoprotein and a component involved in selenoprotein biosynthesis have been identified, the latter with marked impairment of TH homeostasis.

Environment and endocrinology: the case of thyroidology.

Evidence is accumulating for interference of selected endocrine disrupting chemicals (EDC) with the thyroid axis. EDC disturb thyroid hormone (TH) homeostasis leading to developmental defects, hypothyroidism and altered thyroid growth patterns. A rising incidence of papillary thyroid carcinoma (PTC) in several Western countries cannot be definitely accounted for by improved diagnosis or management of thyroid cancer or improved iodine supply. In recent studies, we and others detected, within the thyroid hormone axis, multiple molecular targets of disruption by EDC, which are used in cosmetics, as pesticides or plasticizers or consumed as plant-derived compounds with the diet or with nutritional supplements. Several of these agents exert adverse effects on thyroid growth and function in animal or in vitro cellular models. Major targets are the sodium iodide symporter (NIS), the hemoprotein thyroperoxidase (TPO), the T4 distributor protein transthyretin (TTR), the deiodinases, TH conjugating enzymes and the TR thyroid hormone receptor family. Still prevailing iodine deficiency in many parts of the world predisposes the thyroid gland to adverse effects of endocrine disrupters especially under phases of vulnerability during development and under adaptive challenges during diseases.

Neuronal and ependymal expression of selenoprotein P in the human brain.

Selenoprotein P (SePP) is central to selenium (Se) metabolism in the mammalian organism. Human SePP contains 10 Se atoms that are covalent constituents of the polypeptide chain incorporated as the rare amino acid selenocysteine (Sec). Since hepatocytes secrete SePP into plasma, SePP is commonly regarded as a Se transport protein, although SePP mRNA is expressed in many organs. Gene targeting of SePP in mice leads to neurological dysfunction resulting from Se deficiency and associated reduction of selenoenzyme activities in the brain. However, more recent data revealed that isolated hepatic SePP deficiency does not alter brain Se levels, suggesting a role for SePP locally expressed in the brain. Some of the best characterized and most abundant selenoenzymes, glutathione peroxidases, thioredoxin reductases, and methionine sulfoxide reductase B, play major roles in the cellular defense against reactive oxygen species. Therefore, it was hypothesized that reduced brain Se bioavailability may be involved in the pathogenesis of neurodegenerative disease and normal ageing. We present evidence that human CSF contains SePP and that the human brain expresses SePP mRNA. Moreover, SePP-like immunoreactivity localizes to neurons and ependymal cells and thus appears strategically situated for maintenance and control of Se-dependent anti-oxidative defense systems.

Selenium, the thyroid, and the endocrine system.

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Selenium and selenoproteins in mammals: extraordinary, essential, enigmatic.

Selenium (Se), once known only for its potential toxicity, is now well established as an essential trace element for mammals. Insufficient Se intake predisposes to and manifests in a variety of diseases. Recent studies have proven that it is the synthesis of selenocysteine (Sec)-containing proteins, designated selenoproteins, which represents an essential prerequisite for regular development and a long and healthy life. New transgenic mouse models analysing those selenoproteins with proven enzymatic functions displayed particular phenotypes and highlighted essential Se-dependent processes in development, growth or against specific challenges. While there is a growing molecular understanding of and general agreement on the importance of sufficiently high Se intake and undisturbed selenoprotein biosynthesis, many of the recently identified selenoproteins are still uncharacterised, and the effects and consequences of supra-physiological Se dosages are not biochemically understood. With the recent definition of the human and mouse selenoproteomes and a growing number of available tools, the Se field is now geared for a great leap forward. Se biology has already broadened our knowledge about the genetic code and about protein translation. It now holds great promises also for a better understanding of some key aspects of cancer, inflammation, fertility and prevention of age-associated diseases.

Clinical impact of retinoids in redifferentiation therapy of advanced thyroid cancer: final results of a pilot study.

Differentiated thyroid cancer is a malignant tumour that has a fairly good prognosis, with patients surviving for many years. Multimodal therapy with surgery, radioiodine therapy and TSH suppressive medication is of proven efficacy. However, loss of differentiation is observed in up to one-third of patients with differentiated thyroid cancer, paralleled by an increase in tumour grading and loss of thyroid-specific functions (thyrotropin receptor, iodine accumulation). Such tumours may no longer be amenable to standard treatment protocols, including TSH suppression and radioiodide therapy. Retinoic acids have been shown to exert re-differentiating effects on thyrocytes in various experimental studies and case reports, and it was on this basis that this pilot study was initiated. Patients with advanced thyroid cancer and without the therapeutic options of operation or radioiodide therapy were treated with 13- cis-retinoic acid at a dosage of 1.5 mg/kg body weight daily over 5 weeks. Parameters for assessment of the therapeutic effect were serum thyroglobulin (TG) levels, radioiodine uptake, and tumour size prior to and after retinoid treatment. Fifty patients were evaluated for response, classified as reduction in tumour size and TG levels, stable disease or disease progression. Thirteen patients showed a clear increase in radioiodine uptake, and eight a mild increase. TG levels were unchanged or decreased in 20 patients. Tumour size was assessable in 37 patients; tumour regression was observed in six, and there was no change in 22. In total, a response was seen in 19 patients (38%). Response to retinoid therapy did not always correlate with increased radioiodine uptake, so other direct antiproliferative effects have to be assumed. The encouraging results of the study and the low rate of side-effects with good tolerability of retinoids warrant further studies with altered inclusion criteria and employment of other redifferentiating drugs or combinations of agents.

Modulation of selenoprotein P expression by TGF-beta(1) is mediated by Smad proteins.

Selenoprotein P (SeP) is a selenium-rich plasma protein which accounts for more than 50% this study, the effect of TGF-beta(1) on the expression of SeP in the human liver cell line HepG2 was investigated. Western analysis revealed a dose-dependent reduction of SeP content in cell supernatant. RT-PCR analysis of SeP-mRNA expression demonstrated a marked inhibition and a reporter gene under control of the SeP promoter was negatively regulated by TGF-beta(1). Smad proteins are the transcriptional mediators of TGF-beta signaling. A putative Smad-binding element (SBE) is present in the SeP promoter. In electrophoretic-mobility-shift assays, TGF-beta(1) enhanced the binding of nuclear proteins to this SBE. Overexpression of Smad3 and 4 resulted in a downregulation of SeP-promoter activity whereas deletion of the SBE led to a loss of TGF-beta(1) responsiveness. We conclude that SeP expression is modulated by the binding of Smad3/4 complexes to a functional SBE in the SeP promoter.

The deiodinase family: selenoenzymes regulating thyroid hormone availability and action.

Thyroid hormones control growth, development, differentiation and metabolism in vertebrates. Most of the actions of the active thyroid hormone T3 (3,5,3'-triiodo-L-thyronine) are exerted via ligand-activated nuclear T3 receptors. Activation of the secretory product of the thyroid gland, L-thyroxine (3,3',5,5'-tetraiodo-L-thyronine), or T4, is catalyzed by two enzymes, iodothyronine-5'-deiodinases type I and type II. Inactivation of T4 and T3 occurs via type III iodothyronine-5-deiodinase and to some extent by type I 5'deiodinase. Complementary DNAs (cDNAs) encoding the substrate-binding selenocysteine-containing subunits of the deiodinases were cloned, though some controversy still exists on the type II 5'-deiodinase subunits. Characterization of tissue-specific expression patterns indicates that these selenium-dependent enzymes exert tight control on local and systemic availability of active T3. Thus, deiodinases are envisaged as guardians to the gate of thyroid hormone action mediated by T3 receptors.

[Expression of selenoproteins in monocytes and macrophages--implications for the immune system]. / Expression von Selenoproteinen in Monozyten und Makrophagen--Implikationen für das Immunsystem.

Monocytes differentiate from myeloid precursors towards the macrophage state of differentiation under the influence of 1,25-dihydroxy vitamins D3 (1,25 [OH]2 vitamin D3) and other factors and this is further propagated by colony stimulating factors (MCSF and GMCSF). Macrophage activation and phagocytosis of foreign particles are regularly accompanied by a so called "respiratory burst", an increase in the production of reactive oxygen species (ROS), exerted by the enzyme complex NADPH oxidase. A number of antioxidant enzymes is expressed at the same time to protect the cells from the cytotoxic effects of ROS directed against engulfed microorganisms. The selenium-dependent glutathione peroxidases and thioredoxin reductases are important examples. The cytosolic GPx isoenzyme (cGPx) and thioredoxin reductase alpha (TrxR alpha) are upregulated during the process of differentiation and under the influence of 1.25 (OH)2 vitamin D3. GPx isoenzymes neutralize H2O2. TrxR reduce sulfhydryl-groups like in cysteins either directly or via their cofactor thioredoxin and thus are involved in protein folding and critical protein-protein and protein-DNA interactions, e.g. modulation of dimerization and/or DNA-binding and ligand binding of transcription factors (glucocorticoid receptor and other steroid receptors, NF kappa B). In addition, the antibiotic peptide NK-lysin was shown to be a substrate for TrxR alpha, suggesting that TrxR protects the cell itself from the cytotoxic effects of NK-lysin. Selenium is incorporated into selenocysteine (Secys) in a regulated fashion in the presence of a hairpin structure (Secis element) in the 3'UTR of selenoprotein genes. Secis elements direct the insertion of Secys at UGA codons, which function as opal stop codons in the absence of a suitable Secis element and in selenium deficiency. The above mentioned processes might therefore be altered in relative selenium deficiency or vice versa be upregulated through selenium supplementation. We have shown that TrxR alpha is a 1.25 (OH)2 vitamin D3-responsive early gene in monocytic cells and that TrxR activity as well as GPx activity in these cells can be upregulated by the addition of selenium in vitro and ex vivo. Recent work demonstrates that thioredoxin rapidly enters the cell nucleus upon treatment of cells with H2O2, but little is known about the compartimentalization of the respiratory burst and the intracellular localization of antioxidant enzymes during that process. Macrophage function is insufficient if the generation of a respiratory burst is altered like in hereditary chronic granulomatous disease on one hand, but on the other hand is as well disturbed, if there is a lack in antioxidant enzyme activity. Thioredoxin has been identified as a lymphocyte growth factor and might therefore be involved in the crosstalk between macrophages and lymphocytes. The relevance of the above mentioned and other yet undefined monocytic selenoproteins remains to be elucidated in detail as well as the relevance of selenium supplementation in nutrition in general and in situations of critical infectious disease and autoimmunity.

The trace element selenium and the thyroid gland.

Apart from the essential trace element iodine, which is the central constituent of thyroid hormones, a second essential trace element, selenium, is required for appropriate thyroid hormone synthesis, activation and metabolism. The human thyroid gland has the highest selenium content per gram of tissue among all organs. Several selenocysteine-containing proteins respectively enzymes are functionally expressed in the thyroid, mainly in thyrocytes themselves: three forms of glutathione peroxidases (cGPx, pGPx, and PH-GPx), the type I 5-deiodinase, thioredoxin reductase and selenoprotein P. The thyroidal expression of type II 5-deiodinase still is controversial. As thyrocytes produce H2O2 continuously throughout life an effective cell defense system against H2O2 and reactive oxygen intermediates derived thereof is essential for maintenance of normal thyroid function and protection of the gland. In experimental animal models long-term and strong selenium deficiency leads to necrosis and fibrosis after high iodide loads. Combined iodide and selenium deficiency such as in central Zaire is thought to cause the myxedematous form of endemic cretinism. Inadequate selenium supply and prediagnostically low serum selenium levels are significantly correlated with the development of thyroid carcinoma and other tumors. Though selenium supply controls expression and translation of selenocysteine-containing proteins no direct correlation is found between selenium tissue content and expression of various thyroidal selenoproteins, indicating that other regulatory factors contribute to or override selenium-dependent expression control, e.g., in thyroid adenoma, carcinoma or autoimmune disease. As both trace elements, iodine and selenium, were washed out from the upper layers of the soil during and after the ice ages in many regions of the world adequate supply with these essential compounds needs to be provided either by a balanced diet or supplementation.

Expression pattern of gastrointestinal selenoproteins--targets for selenium supplementation.

There is experimental and epidemiological evidence for an association between low selenium levels and gastrointestinal cancer incidence, prevalence, and mortality. To identify targets for selenium supplementation in the human digestive tract, we examined mRNA expression of various selenocysteine-containing proteins in normal mucosa biopsy specimens. Tissue samples from the esophagus and from different sites of the stomach, small bowel, and colon were obtained during endoscopies of the upper and lower gastrointestinal tract. Northern blot analyses revealed a lack of cytosolic glutathione peroxidase mRNA but a differential mRNA expression pattern of gastrointestinal and plasma glutathione peroxidase, selenoprotein P, and thioredoxin reductase. Glutathione peroxidase and thioredoxin reductase activities were detected in the mucosa of all biopsies, but the differential pattern did not reflect the differential mRNA steady-state levels. In addition to gastrointestinal glutathione peroxidase, which was found to play a role in colon cancer resistance, we identified further gastrointestinal selenoproteins, which may be involved in gastrointestinal cell defense and cell differentiation.

[Selenoproteins in bone, gastrointestinal tract and thyroid gland of the human]. / Selenoproteine im Knochen, Gastrointestinaltrakt und in der Schilddrüse des Menschen.

BASIS: Selenium is an essential trace element, which is incorporated as selenocysteine (secys) into specific proteins in a regulated fashion. In the presence of a hairpin loop structure within the 3' untranslated region of the mRNA the opal stop codon UGA is coding for selenocysteine. Selenoprotein functions are dependent on secys incorporation. Members of the family of deiodinases as well as the family of glutathione peroxidases, selenoprotein P and thioredoxin reductase are selenoproteins. DISCUSSION: Bone, the intestine and the thyroid rely on antioxidant systems against potential cell and DNA damage through endogenous and environmental peroxides and reactive oxygen species (ROS) potentially promoting inflammation and tumorigenesis. Optimized cell defense through antioxidant selenoproteins requires optimal selenium supplementation of the organism. We have analyzed the expression of selenoproteins in these tissues, thus providing molecular tools to further elucidate optimal selenium supply on a cellular level. CONCLUSION: Clinical intervention studies that focus on the development of disease must confirm the relevance of optimized selenium supply for the pathogenesis, prevention and therapy of metabolic bone disease as well as chronic (autoimmune) inflammation and tumorigenesis in the thyroid and intestine.

Thyroid hormone deiodinases--a selenoenzyme family acting as gate keepers to thyroid hormone action.

Development and tissue-specific deiodination of thyroid hormone leads to both activation of the prohormone thyroxine to the thyromimetically active T3 as well as to inactivation of T3 and its conjugates or inactivation of T4 to yield potential regulatory active rT3. At least three deiodinase isoenzymes have so far been characterized and cloned, and the deiodinase isozymes represent a new family of eukaryotic selenoproteins for which an enzyme function could be assigned. Selenium status apparently regulates the expression of these deiodinase isozymes to different extent indicating that a hierarchy of selenium incorporation exists for those enzymes. Currently, it appears that selenium deficiency does not affect expression of type II 5'-deiodinase or 5-deiodinase to a marked extent in vivo whereas type I 5'-deiodinase at least in liver and kidney is reduced in severe selenium deficiency. However, daily selenium intake in normal mideuropeans already saturates the requirement for the expression of the deiodinase isoenzymes. So far, only reduced expression of 5'-D I and decreased T 3 production has been observed in specific diets such as for PKU or in cystic fibrosis, where transport of ions (iodide, selenite?) might be affected. Further alterations of T3 production by 5'-D I activity are observed under the conditions of the low T3 syndrome, which comprise a broad spectrum of clinical disorders from carbohydrate withdrawal to intensive care patients. It is not yet clear if selenium supplementation or T3 treatment is beneficial to these patients. The marked tissue-specificity of expression of the deiodinases requires more detailed examinations on the relation between these enzymes and the expression of thyroid hormone action, which is mediated by the nuclear T3 receptor family or receptors and signal transduction molecules in the mitochondria, plasma membrane, or cytoskeleton. The location of the deiodinase enzymes either at the inner side of the plasma membrane or the cytosolic side of the endoplasmic reticulum positions these enzymes to a strategically important location enabling them to act as gate-keepers to the nuclear receptors. Similar to other enzymes involved in the activation or inactivation of compounds with hormone or signalling function, the deiodinases are key elements in the intracrine regulation of hormone activation in target tissues or inactivation in non-target tissues. Therefore, a detailed molecular, cell biological and physiological analysis of the function, regulation and gene structure of these enzymes is required before a development of tissue- or enzyme-specific pharmacological intervention is possible. Nevertheless, first data indicate that reduced 5'-deiodinase type I expression in tumor tissues can be re-induced by treatment with retinoids at least in follicular thyroid carcinoma. Further studies are needed to prove that retinoids might be a useful therapeutic tool for re-differentiation therapy of thyroid carcinoma which are inaccessible to surgical intervention or lack radio-iodide uptake and storage. The important function and regio- and cell-specific expression of deiodinase isozymes in the central nervous system is far from being understood. Current first evidence suggests a close interaction between thyroid hormone deiodination, thyroid hormone concentration, and expression of thyroid hormone responsive genes in the adult brain as well as tight regulation and interaction between thyroid hormone metabolism and neurotransmitter synthesis release and action.

Binding of thyroxine to pig transthyretin, its cDNA structure, and other properties.

Thyroxine binding to proteins in pig plasma during electrophoresis was observed in the albumin, but not in the prealbumin and post-albumin regions. Transthyretin could be identified in medium from in vitro pig choroid plexus incubations by size and number of subunits and a very high rate of synthesis and secretion. Its electrophoretic mobility was intermediate between that of thyroxine-binding globulin and albumin. It bound thyroxine, retinol-binding protein, anti-(rat transthyretin) antibodies and behaved similarly to transthyretins from other vertebrate species when plasma was extracted with phenol. Inhibition experiments with the synthetic flavonoid F 21388, analysing the binding of thyroxine, suggested that transthyretin is not a major thyroxine carrier in the bloodstream of pigs. Cloning and sequencing of transthyretin cDNA from both choroid plexus and liver showed that the same transthyretin mRNA is expressed in pig choroid plexus and liver. The amino acid sequence derived from the nucleotide sequence revealed that pig transthyretin differs from the transthyretins of all other studied vertebrate species by an unusual C-terminal extension consisting of the amino acids glycine, alanine and leucine. This extension results from the mutation of a stop codon into a codon for glycine. The unusual C-terminal extensions do not seem to interfere with the access of thyroxine to its binding site in the central channel of transthyretin.

Differential selenium-dependent expression of type I 5'-deiodinase and glutathione peroxidase in the porcine epithelial kidney cell line LLC-PK1.

The Se-dependent expression of two selenoproteins, cytosolic glutathione peroxidase (cGPx) and type I iodothyronine-5'-deiodinase (5'DI), was investigated in the porcine epithelial kidney cell line LLC-PK1 in serum-free medium. The selenite-dependent expression of cGPx and 5'DI was revealed by enzyme-activity measurements, affinity labelling of 5'DI, metabolic labelling of proteins with 75Se and steady-state mRNA analysis. The expression of the two enzymes strongly depended on selenite concentrations of the culture medium. cGPx required 2-fold higher selenite levels than 5'DI to reach half-maximal activity. The Se-dependent enzyme activities were approximately paralleled by the corresponding steady-state mRNA levels. The response of the two enzymes to Se supply was further characterized by kinetic Se-depletion and -repletion experiments. Upon removal of medium selenite, cGPx activity decreased exponentially, whereas after an initial decrease over 1-2 days, 5'DI levels completely recovered during a further 2 days. These data indicate a differential Se-dependent regulation of the two selenoproteins, with 5'DI being preferentially supplied with the trace element Se, thus ensuring a continuous cellular capacity for thyroid-hormone activation, even under Se-deficient conditions. The abundant cGPx in cells with sufficient Se supply might serve as a cellular Se store which can be mobilized for the synthesis of more vital selenoproteins such as 5'DI under shortage conditions. Thus, a cellular hierarchy of selenoprotein expression, reflected by different individual regulation mechanisms at the transcriptional and post-transcriptional level, adds to the previously recognized tissue-specific hierarchy of Se retention.

Cloning of a cDNA encoding an ectoenzyme that degrades thyrotropin-releasing hormone.

Thyrotropin-releasing hormone (TRH) is an important extracellular signal substance that acts as a hypothalamic-releasing factor, which stimulates the release of adenohypophyseal hormones and functions as a neurotransmitter/neuromodulator in the central and peripheral nervous system. The inactivation of TRH after its release is catalyzed by an ectoenzyme localized preferentially on neuronal cells in the brain and on lactotrophic pituitary cells. This enzyme exhibits a very high degree of substrate specificity as well as other unusual properties. The activity of the adenohypophyseal enzyme is stringently controlled by estradiol and thyroid hormones, indicating that this enzyme itself may serve regulatory functions. Fragments of the enzyme isolated from rat or pig brain were generated by enzymatic digestion or cyanogen bromide cleavage, purified by reverse-phase HPLC, and sequenced. PCR amplification and screening of cDNA libraries from rat brain and pituitary led to the identification and isolation of a cDNA that encodes a protein of 1025 amino acids. The analysis of the deduced amino acid sequence was consistent with the identification of the enzyme as a glycosylated, membrane-anchored Zn metallopeptidase. Furthermore, Northern blot analysis demonstrated that the mRNA levels paralleled the tissue distribution of the enzyme and that in pituitary tissue the transcript levels rapidly increased when the animals were treated with triiodothyronine. Finally, transient transfection of COS-7 cells with this cDNA led to the expression of an active ectopeptidase that displayed the characteristics of the TRH-degrading ectoenzyme.

Thyroid hormone deiodination in target tissues--a regulatory role for the trace element selenium?

The three isozymes catalyzing deiodination of thyroid hormones and iodothyronines derived thereof exert a major role in tissue- and development-specific expression of thyroid hormone action in target tissues by activating the prohormone T4 to thyromimetically active T3 or by inactivating the prohormone T4 or active T3 in non-target tissues at inappropriate time points. These three isozymes, in cooperation with the enzymes responsible for non-deiodinative degradation of iodothyronines, thus act as "guardians of the gate" to nuclear thyroid hormone receptors and other cellular target sites for thyroid hormone action. Strict and distinct hormonal, nutritional and nerval regulation of expression of the deiodinase isozymes warrants a closely coordinated control of thyroid hormone action, which directs growth, differentiation, and basal metabolic functions in the developing and the adult organism both in the periphery and in the central nervous system. The integrative action of this essential homeostatic and dynamic ancient hormone system in higher vertebrates is under the influence of two essential trace elements, iodine and selenium, which both are inadequately available for man and life stock in great parts of the world. As soon as, and only if, iodine supplementation is achieved, attempts to establish adequate selenium supply for thyroid hormone synthesis, activation, metabolism and action should be made, but not the other way around. In this review, the physiological, biochemical and pharmacological properties of the three deiodinase isozymes are discussed in detail, with special attention to the role of selenium in regulation of type I 5'-deiodinase expression. The contribution of each deiodinase izozyme to the activation and inactivation of thyroid hormones in specific tissues is reviewed.

Selenium-dependent regulation of type I 5'-deiodinase expression.

Selenite concentration regulates activity and expression of the p27 substrate-binding subunit of type I 5'deiodinase (5'-D) and of a protein labeled with bromoacetylthyroxin (BrAcT4), or p30, with yet unknown function in a porcine-kidney epithelial cell line (LLC-PK1) cultured in serum-free medium. p27 is metabolically labeled by 75-selenite and affinity labeled by BrAc[125I]T4. Compared with glutathione peroxidase, expression of the p27 5'D subunit (5'-DI) is observed at 10-fold-lower concentrations of selenium in the growth medium, suggesting an intracellular hierarchy of selenite utilization. Selenium deficiency retards cell growth and prevents 5'-DI expression and may thus impair thyroid hormone action in vivo.